1. Field of the Invention
The present invention relates to an engine and a straddled vehicle.
2. Description of the Related Art
A DOHC (Double Over Head Camshaft) type engine is used as an engine required to generate a high rotation speed and high output. In the DOHC type engine described in JP 2011-32925 A, an intake-side camshaft and an exhaust-side camshaft that respectively drive an intake valve and an exhaust valve are provided. An intake-side sprocket and an exhaust-side sprocket that receive a rotational force of a crankshaft are respectively provided at one end of the intake-side camshaft and one end of the exhaust-side camshaft. The size of a cylinder head is likely to increase due to such a configuration.
It might be considered that a distance between the intake-side camshaft and the exhaust-side camshaft is reduced in order to prevent an increase in size of the above-mentioned engine. However, in the above-mentioned cylinder head, a support that rotatably supports the intake-side camshaft (hereinafter referred to as an intake-side support) and a support that rotatably supports the exhaust-side camshaft (hereinafter referred to as an exhaust-side support) are provided. Further, a plug hole through which an ignition plug is inserted is provided between the intake-side support and the exhaust-side support. Therefore, when the distance between the intake-side camshaft and the exhaust-side camshaft is reduced, the intake-side support and the exhaust-side support interfere with the ignition plug. Therefore, it is difficult to reduce the distance between the intake-side camshaft and the exhaust-side camshaft.
On the other hand, it might be considered that the intake-side camshaft and the exhaust-side camshaft are located closer to a cylinder body in order to prevent an increase in the size of the above-mentioned engine. However, an intake port and an exhaust port extending from a combustion chamber that is inclined with respect to a cylinder axis are provided in the above-mentioned cylinder head. If the distance between the intake-side camshaft and the exhaust-side camshaft cannot be reduced, a valve included angle between the intake valve and the exhaust valve cannot be reduced. In this case, when components for respectively driving the intake valve and the exhaust valve are located closer to the cylinder body, these components interfere with the intake port and the exhaust port. Therefore, it is difficult to arrange the intake-side camshaft and the exhaust-side camshaft closer to the cylinder body.